1. Field of the Invention
The present invention relates to an image formation apparatus and method to form an image through electrophotography based on digital image information. Particularly, the present invention relates to an image formation apparatus and method forming a halftone image through exposure energy modulation.
2. Description of the Background Art
As to the output image of an image output equipment utilizing electrophotography such as a digital copy machine or printer, one continuing goal is to improve the picture quality of high definition close to that of a silver halide photograph. Essential requirements to realize a picture quality equal to that of a photograph includes quite a little gray scale levels to represent a halftone image, increase in the number of gray levels particularly in the low density region (improvement of the tone resolution), improvement in reproducibility, reduction in graininess (asperity in image) and the like. It is expected that a halftone image of good quality having a smooth gray scale property can be realized by meeting the above requirements.
Various conventional methods in tone rendition are known, including the bi-tonal recording system such as the dither method, density pattern method and error diffusion method, as well as the multilevel recording system such as the laser intensity modulation method and laser pulse width modulation method.
The dither method which is one of the gray scale reproduction technique compares a threshold value calculated at a predetermined rule for each pixel with the multiple gray level of an input pixel within a dither matrix formed of a plurality of pixels in an input image to determine black and white.
The density pattern method reproduces the gray scale by setting one pixel in the input image to correspond to a matrix of nxc3x97n pixels, and control the number of pixels to be printed out in the matrix according to the density level of each pixel in the input image.
All of these methods are characterized in that the number of gray levels that can be reproduced is increased by setting a larger matrix size. These methods are particularly suitable to improve the gray scale reproducibility in a low density region.
However, these methods have the disadvantage that the output resolution will be degraded by increasing the matrix size. Therefore, minimizing reduction in the output resolution in addition to maintaining favorable gray scale reproduction in the low density region have been the conventional issues.
In view of these issues, Japanese Patent Laying-Open No. 61-5676, for example, discloses the approach of preparing a plurality of matrix patterns that have a larger matrix size in proportion to a lower level of the multiple gray level to enable recording of a low density region without degrading the resolution.
According to the art disclosed in this publication, the distance between adjacent dots can be increased by using a large-sized matrix at the low density region, whereby the apparent density can be reduced. At the high density region, the matrix size is set smaller. Therefore, the output resolution can be maintained.
However, the method disclosed in Japanese Patent Laying-Open No. 61-5676 cannot provide subtle density control as compared to the method of controlling the size of one dot through exposure energy modulation. Therefore, the number of gray levels that can be reproduced is limited. It was difficult to obtain a smooth gray scale rendition at a low density area where small dots are required.
Even if the low density area can be reproduced by forming dots of relative large size to take a large matrix size, graininess was exhibited in some cases depending upon the dot diameter. There was a possibility that the picture quality is adversely degraded.
Japanese Patent Laying-Open No. 2-39957 discloses the technique of recording an image of wide gray scale width without degrading the resolution by altering the recording dot size and dot pattern. According to this method, an image of a low density region that could not be achieved by just altering the dot diameter can be realized.
However, the method disclosed in Japanese Patent Laying-Open No. 2-39957 has no restriction in the dot diameter. In the case where a pattern of few dots directed to represent a low density area among the various pixel patterns is applied, it is expected that significant graininess will be exhibited when a certain dot size is exceeded. In such a case, the picture quality of the output image will be eventually degraded.
An object of the present invention is to provide an image formation apparatus and image formation method that can have graininess that depends upon the correlation between the dot-dot distance and dot diameter reduced to realize a favorable gray scale rendition particularly in the low density region.
According to an aspect of the present invention, an image formation apparatus includes an exposure unit, a development unit, a storage unit, and a processing unit.
The exposure unit applies a laser beam on a photoconductor charged to a predetermined potential at a predetermined polarity to form an electrostatic latent image. The development unit renders the electrostatic latent image visible by applying the developer electrostatically. The storage unit prestores a plurality of exposure patterns having dots scattered and arranged with the distance between adjacent dots set constant in both the main scanning direction and the subscanning direction, a first table in which the density range that can be reproduced by each exposure pattern is stored, and a second table indicating the correlation between the dot diameter and output density of each exposure pattern. The processing unit provides control of the exposure unit when a halftone image region that is below a predetermined density is to be reduced. In reproducing a halftone image region that is below a predetermined density, the processing unit i) selects an exposure pattern corresponding to an input density value by referring to the first table, ii) in the case where a plurality of exposure patterns are selected for the input density value, deriving the dot diameter required for each of the plurality of exposure patterns by referring to the second table, and selecting an exposure pattern that satisfies the relation of D/Nxe2x89xa60.4 where D is the dot diameter and N is the number of dots present per 1 inch for each of the plurality of exposure patterns, and iii) controlling the exposure unit using the selected exposure pattern.
In the operation of reproducing a halftone image region that is below a predetermined density, an optimum exposure pattern that allows favorable reproducibility without graininess in the output image can be selected from a plurality of exposure patterns having dots scattered and arranged with the distance between adjacent dots set constant in both the main scanning direction and subscanning direction.
Preferably, the processing unit functions to select an exposure pattern that has the lowest dot count N per 1 inch when there are a plurality of exposure patterns that satisfies the relation.
Since the selected exposure pattern has a large required dot diameter, variation in the dot diameter in forming the exposure pattern can be suppressed to a low level. In other words, dots can be formed in superior reproducibility and stably.
Preferably, the number of dots N present per 1 inch in the exposure pattern that has the largest distance between adjacent dots satisfies the relation of 75xe2x89xa6Nxe2x89xa6150 among the plurality of exposure patterns.
By employing an exposure pattern satisfying the relationship of 75xe2x89xa6Nxe2x89xa6150 where N is the dot count per 1 inch for an image region of extremely low density, a halftone image suppressed in graininess and of favorable reproducibility can be formed.
Preferably, the image formation apparatus reproduces the gray scale by a combination of the exposure pattern and dot diameter control through exposure energy modulation. The storage unit also stores a third table that indicates the correlation between an exposure energy and a dot diameter. The processing unit derives the required dot diameter in the determined exposure pattern by referring to the second table for the image density level of each pixel forming the halftone image region. The processing unit includes an exposure energy density determination unit deriving the exposure energy required to obtain the derived dot diameter by referring to the third table. The image formation apparatus further includes a laser driver that effects exposure so as to obtain the desired output density by controlling the exposure unit based on the determined exposure pattern and exposure energy.
By controlling the exposure energy quantitatively based on the second table storing the correlation between the dot diameter and output density for each of the plurality of exposure patterns and the third table storing the correlation between the exposure energy and the dot diameter, the desired density can be reproduced at favorable accuracy by any of the exposure pattern.
Preferably, the exposure energy modulation is the pulse width modulation controlling the pulse width of the laser drive pulse for each dot.
By supplying the drive pulse having the pulse width controlled according to the required exposure energy density to the light source, the light source can be driven at a drive energy optimal to the dots that form the halftone image. Since the optimum exposure energy can be applied to the dots, favorable dot reproducibility can be achieved.
Preferably, the exposure energy modulation is the intensity modulation controlling the applied energy for each dot.
By supplying a drive pulse having the input energy controlled according to the required exposure energy density to the light source, the light source can be driven at a drive energy optimal to the dots that form the halftone image. Since the optimum exposure energy can be applied to the dots, favorable dot reproducibility is achieved.
According to another aspect of the present invention, an image formation method of gray scale rendition includes the steps of: obtaining the density value of a pixel of interest, comparing the density value with a predefined density value; selecting an exposure pattern corresponding to the input density value by referring to a first table prestored with the density range that can be reproduced by each of a plurality of predetermined exposure patterns when reproducing a halftone image region that is below a predetermined density; deriving a dot diameter required for each of the plurality of exposure patterns by referring to a second table indicating the correlation between the dot diameter and output density of each exposure pattern when a plurality of exposure patterns are selected for the input density value; and selecting an exposure pattern satisfying the relation of D/Nxe2x89xa60.4 where D is the derived dot diameter and N is the number of dots present per 1 inch.
In reproducing a halftone image region that is below a predetermined density, the first table stored with the density range that can be reproduced by each of a plurality of preset exposure patterns is accessed. Then, an exposure pattern corresponding to the input density value is selected. When a plurality of exposure patterns are selected for the input density value, the second table indicating the correlation between the dot diameter and output density of each exposure pattern is accessed to derive the dot diameter required for each of the plurality of exposure patterns. Then, an exposure pattern that satisfies the relation of D/Nxe2x89xa60.4 where D is the derived dot diameter and N is the number of dots present per 1 inch for each of the plurality of exposure patterns is selected. By this series of processes, the graininess can be reduced for the realization of representing favorable gray scale.
Preferably, the exposure pattern has dots scattered and arranged with the distance between adjacent dots set constant in both the main scanning direction and the subscanning direction.
Since the pattern is arranged symmetrically in the vertical and horizontal direction, the patterns can be joined agreeably in the vertical and horizontal direction. It is to be noted that each pattern can be reproduced by magnifying or reducing with a constant scale in the main scanning direction and the subscanning direction. Therefore, by storing just one basic pattern instead of storing all the plurality of patterns in the storage unit, the other patterns can be generated by multiplying by a preset scale rate. Thus the amount to be stored in the storage unit can be reduced.
Preferably, in the case where there a plurality of exposure patterns that satisfy the relationship of D/Nxe2x89xa60.4, an exposure pattern with the lowest number of dots N present per 1 inch is selected.
Since the exposure pattern with the lowest dot count N is an exposure pattern with the largest required dot diameter, variation in the dot diameter when an exposure pattern is to be formed can be suppressed. Thus, dots can be formed in favorable reproducibility and stably.
Furthermore, the problem of not being able to identify an exposure pattern by an algorithm of selecting a single pattern due to the presence of a plurality of exposure patterns can be obviated.
Preferably, the number of dots N present per 1 inch in the exposure pattern that has the largest distance between adjacent dots among the plurality of prestored exposure patterns satisfies the relation of 75xe2x89xa6Nxe2x89xa6150.
Thus, the halftone region can be reproduced in further fidelity in the case where the density value is extremely low such as in the range of, for example, 0.1 to 0.3.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.